1. Field of the Invention
The present invention relates to a novel method for producing ditrimethylolpropane (hereinafter referred to as “di-TMP”).
2. Description of the Prior Art
Highly pure di-TMP has been used as the raw materials for the production of polyacrylate, polyether polyol, polyurethane, alkyd resin, synthetic lubricating oil, etc. di-TMP is by-produced and recovered in the industrial production of trimethylolpropane (hereinafter referred to as “TMP”) by aldol condensation and crossed Cannizzaro reaction of n-butyl aldehyde (hereinafter referred to as “NBD”) and formaldehyde (for example, U.S. Pat. No. 3,097,245). In the proposed method, a TMP extract (crude TMP) containing substantially no sodium formate is obtained by extracting the reaction product solution of the reaction of NBD and formaldehyde with a solvent, after condensing or without condensing the reaction product solution. The purification of the crude TMP by high vacuum distillation leaves a still residue containing 1 to 20% of TMP and 20 to 50% di-TMP. To recover di-TMP from the still residue, there have been proposed crystallization from ethyl acetate (for example, Japanese Patent Application Laid-Open No. 47-30611), crystallization from water in the presence of sodium formate (for example, Japanese Patent Application Laid-Open No. 49-133311), crystallization from 1,4-dioxane (for example, Japanese Patent Application Laid-Open No. 2002-47231), etc.
However, only by the crystallization from the organic solvent such as ethyl acetate, a highly pure di-TMP is not obtained from the distillation still residue in the production of TMP. When the distillation still residue is colored, the contamination of di-TMP with colored substances is not prevented in the crystallization from water. Therefore, to obtain a highly pure di-TMP, the crystallizing operation should be repeated in these methods.
It is reported that the crystallization from 1,4-dioxane enables the recovery of a highly pure di-TMP only in one crystallizing operation. However, 1,4-dioxane is toxic and easily converted into explosive peroxide by the reaction with oxygen in air. Therefore, it is desirable to avoid the use of 1,4-dioxane as much as possible in the industrial process, because there is a danger of explosion during the recovery of 1,4-dioxane for reuse by distilling the filtrate after the crystallization.